Re: + stackdepot-fix-mempolicy-use-after-free.patch added to -mm tree

[David Rientjes <rientjes@google.com>]

On Fri, 19 Aug 2016, akpm@linux-foundation.org wrote:

> From: Vegard Nossum <vegard.nossum@oracle.com>
> Subject: stackdepot: fix mempolicy use-after-free
> 
> This patch fixes the following:
> 
>     BUG: KASAN: use-after-free in alloc_pages_current+0x363/0x370 at addr ffff88010b48102c
>     Read of size 2 by task trinity-c2/15425
>     CPU: 0 PID: 15425 Comm: trinity-c2 Not tainted 4.8.0-rc2+ #140
>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.3-0-ge2fc41e-prebuilt.qemu-proje
>     ct.org 04/01/2014
>      ffff88010b481040 ffff88010b557650 ffffffff81f08d11 ffff88011a40d380
>      ffff88010b481028 ffff88010b557678 ffffffff815dac7c ffff88010b557708
>      ffff88010b481028 ffff88011a40d380 ffff88010b5576f8 ffffffff815daf15
>     Call Trace:
>      [<ffffffff81f08d11>] dump_stack+0x65/0x84
>      [<ffffffff815dac7c>] kasan_object_err+0x1c/0x70
>      [<ffffffff815daf15>] kasan_report_error+0x1f5/0x4c0
>      [<ffffffff815db2fe>] __asan_report_load2_noabort+0x3e/0x40
>      [<ffffffff815cb903>] alloc_pages_current+0x363/0x370    <---- use-after-free
>      [<ffffffff81fa9954>] depot_save_stack+0x3f4/0x490
>      [<ffffffff815d9bb5>] save_stack+0xb5/0xd0
>      [<ffffffff815da211>] kasan_slab_free+0x71/0xb0
>      [<ffffffff815d6643>] kmem_cache_free+0xa3/0x290
>      [<ffffffff815c8149>] __mpol_put+0x19/0x20               <---- free
>      [<ffffffff81260635>] do_exit+0x1515/0x2b70
>      [<ffffffff81261dc4>] do_group_exit+0xf4/0x2f0
>      [<ffffffff81281c5d>] get_signal+0x53d/0x1120
>      [<ffffffff8119e993>] do_signal+0x83/0x1e20
>      [<ffffffff810027af>] exit_to_usermode_loop+0xaf/0x140
>      [<ffffffff810051e4>] syscall_return_slowpath+0x144/0x170
>      [<ffffffff83ae406f>] ret_from_fork+0x2f/0x40
>     Read of size 2 by task trinity-c2/15425
> 
> The problem is that we may be calling alloc_pages() in a code path where
> current->mempolicy has already been freed.
> 
> By passing __GFP_THISNODE we will always use default_mempolicy (which
> cannot be freed).
> 
> Link: https://lkml.org/lkml/2016/7/29/277
> Link: https://github.com/google/kasan/issues/35
> Link: http://lkml.kernel.org/r/1471603265-31804-1-git-send-email-vegard.nossum@oracle.com
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
> Acked-by: Dmitry Vyukov <dvyukov@google.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  lib/stackdepot.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff -puN lib/stackdepot.c~stackdepot-fix-mempolicy-use-after-free lib/stackdepot.c
> --- a/lib/stackdepot.c~stackdepot-fix-mempolicy-use-after-free
> +++ a/lib/stackdepot.c
> @@ -243,6 +243,12 @@ depot_stack_handle_t depot_save_stack(st
>  		alloc_flags &= ~GFP_ZONEMASK;
>  		alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
>  		alloc_flags |= __GFP_NOWARN;
> +		/*
> +		 * Avoid using current->mempolicy which may already have
> +		 * been freed -- we may be in the process of saving the
> +		 * stack for exactly that __mpol_put() call.
> +		 */
> +		alloc_flags |= __GFP_THISNODE;
>  		page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
>  		if (page)
>  			prealloc = page_address(page);

This is wrong, it unnecessarily restricts the allocation to a local node 
and has a greater chance to fail.  Passing __GFP_THISNODE here is only an 
implementation detail of mempolicies to avoid reference to freed policy.  
It is easy to fix by using alloc_pages_node(NUMA_NO_NODE, alloc_flags, 
STACK_ALLOC_ORDER) instead of alloc_pages() directly.